UNIT 3: WIRELESS NETWORKS - EXAM-FOCUSED SHORT NOTES

I. CELLULAR MOBILE COMMUNICATION SYSTEMS (2G/3G/4G/5G EVOLUTION)

A. GSM to UMTS Evolution

• GSM (2G): Circuit-switched core, TDMA/FDMA air interface (200 kHz channels), ~9.6 kbps data (HSCSD), voice + SMS focus.

• Evolution Drivers: Demand for higher data rates (mobile internet), support for packet-switched services, improved spectral efficiency.

• UMTS (3G): Introduced WCDMA (CDMA-based, 5 MHz carriers), packet-switched core (GPRS/EDGE演进), peak rates ~2 Mbps (stationary), support for multimedia services.

[!TIP] Exam often asks for comparison table. Key differentiators: Access Tech (TDMA vs CDMA), Data Rate, Core Network (Circuit vs Packet), Services (Voice/SMS vs Mobile Internet).

B. Universal Mobile Telecommunications System (UMTS)

Network Architecture:

1. Core Network (CN):

   • Circuit-Switched (CS): MSC (call control), VLR (visitor location register), HLR (home location register).

   • Packet-Switched (PS): SGSN (serving GPRS support node - mobility, routing), GGSN (gateway - external network interface).

2. UTRAN (UMTS Terrestrial Radio Access Network):

   • Node B: Base station (radio transmission/reception).

   • RNC (Radio Network Controller): Manages radio resources, handovers, connects Node Bs to CN.

Functional Interaction: CS services use MSC/VLR; PS services use SGSN/GGSN. RNC controls radio link setup/management. HLR/VLR handle subscriber location/authentication.

Air Interface: WCDMA (Wideband CDMA). Supports FDD (frequency division duplex, paired spectrum) and TDD (time division duplex, unpaired spectrum).

C. 3rd Generation Partnership Project (3GPP)

• Role: Global collaboration (ARIB, ATIS, CCSA, ETSI, TSDSI, TTA, TTC) to develop technical specifications for mobile systems (GSM, UMTS, LTE, 5G).

• Objectives: Ensure global compatibility, interoperability, and seamless evolution. Defines standards for radio, core network, service architecture.

• Key for LTE/LTE-A: 3GPP Release 8 (LTE foundation), Release 10 (LTE-Advanced - carrier aggregation, enhanced MIMO).

D. Long-Term Evolution (LTE) & E-UTRAN Architecture

Simplified Flat All-IP Architecture:

UE → eNodeB (eNB) → S-GW → P-GW → Internet

                ↑

                MME (Control Plane)

• eNodeB (eNB): Single node (RNC eliminated). Functions: Radio resource management, header compression, encryption, connectivity to MME/S-GW.

• Mobility Management Entity (MME): Control plane only. Handles authentication, bearer management, idle mode tracking, handover signaling.

• Serving Gateway (S-GW): User plane. Local mobility anchor, data routing/forwarding, lawful interception.

• Packet Data Network Gateway (P-GW): User plane. IP address allocation, policy enforcement, charging, connects to external PDNs.

Key Features:

• All-IP (no circuit switching).

• OFDMA (downlink), SC-FDMA (uplink).

• MIMO support (up to 4x4).

• Low latency (<10 ms), flexible bandwidth (1.4-20 MHz).

E. IEEE 802.16 (WiMAX)

• Fixed WiMAX (802.16-2004): Point-to-multipoint, fixed stations, licensed bands.

• Mobile WiMAX (802.16e): Added mobility support:

  • Hard Handover: Break-before-make (simpler).

  • Fast Handover: MAC-layer, predictive, network-assisted.

  • Sleep Mode & Power Management: To conserve battery in mobiles.

  • Scalable OFDMA: Adaptive to different channel bandwidths.

• Architecture: Base Station (BS), Subscriber Station (SS), Relay Station (RS). Comparison with Cellular: Similarities (BS, SS, handover), Differences (more flexible bandwidth, different air interface).

II. WIRELESS LOCAL/PERSONAL AREA NETWORKS (WLAN/WPAN)

A. IEEE 802.11 WLAN Standards

Protocol Architecture (Layered):

Application

  ↑

LLC (Logical Link Control) - Common to all 802.x

  ↑

MAC (Medium Access Control) - 802.11 Specific

  ↑

PHY (Physical Layer) - Variants: FHSS, DSSS, OFDM (a/g/n/ac/ax)

MAC Layer Deep Dive:

• CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance):

  • DCF (Distributed Coordination Function): Mandatory, contention-based. Uses DIFS + random backoff.

  • PCF (Point Coordination Function): Optional, contention-free (polling by AP).

• Frame Formats:

  • Management Frames: Association, authentication, beacons.

  • Control Frames: RTS, CTS, ACK.

  • Data Frames: Carry payload.

• Association/Authentication: Station discovers AP (beacon), sends association request, authenticates (open/WEP/WPA).

• Medium Access Problems & Solutions:

  • Hidden Terminal Problem: Station A & C cannot hear each other, both transmit to B → collision at B.

    • Solution: RTS/CTS (Request-to-Send / Clear-to-Send) handshake. RTS from A → CTS from B (heard by C) → C defers.

  • Exposed Terminal Problem: Station B hears A transmitting to AP, so B defers even though its intended receiver (C) is out of A's range → unnecessary deferral.

    • Solution: CTS-to-Self (in 802.11g/n), directional antennas.

Physical Layer Overview:

• FHSS (Frequency-Hopping Spread Spectrum): 802.11 (original).

• DSSS (Direct-Sequence Spread Spectrum): 802.11b (2.4 GHz, 11 Mbps).

• OFDM (Orthogonal Frequency Division Multiplexing): 802.11a/g/n/ac/ax. Uses multiple orthogonal subcarriers, robust against multipath.

B. HIPERLAN (High-Performance Radio LAN)

• Key Features vs. 802.11:

  • Connection-Oriented: Uses CH (Connection Handler) for QoS.

  • Multihop Support: Can form ad-hoc mesh networks natively.

  • Dynamic TDMA/TDD: More efficient than CSMA/CA for QoS.

  • Higher Data Rate: Up to 54 Mbps (HIPERLAN/2).

• HIPERLAN/2 Structure: Similar to cellular (central controller - AP), supports QoS classes (constant/variable bit rate).

C. IEEE 802.15 WPAN & Related Technologies

1. IEEE 802.15.1 (Bluetooth):

• Piconet: Master (max 7 active slaves) + 255 parked slaves. Frequency Hopping Spread Spectrum (FHSS) across 79 channels (2.4 GHz).

• Scatternet: Interconnected piconets. A device can be master in one, slave in another. Increases coverage area and total nodes, but complex synchronization, reduced per-piconet throughput.

• Device Interaction: Master controls timing, polling slaves. Slaves only transmit when polled.

2. IEEE 802.15.4 (Zigbee):

• Low-Rate WPAN: 250 kbps (2.4 GHz), 40 kbps (915 MHz), 20 kbps (868 MHz). Ultra-low power, low cost.

• Topologies: Star (simple), Cluster-Tree (hierarchical), Mesh (robust, multi-hop).

• Applications: IoT, sensor networks, home automation.

3. 802.11 & Bluetooth Interface (Coexistence):

• Challenge: Both operate in 2.4 GHz ISM band → co-channel interference.

• Mechanisms:

  • Adaptive Frequency Hopping (AFH): Bluetooth avoids 802.11 channels in use.

  • Packet Scheduling: Time-division coordination.

  • Physical Separation: Using different channels (802.11 channels 1,6,11 are non-overlapping).

D. Wireless ATM

• Concept: Extend ATM cell-switching to wireless links. Provide QoS guarantees (CBR, VBR) over variable wireless channels.

• Architecture: Mobile Terminal → Base Station → W-ATM Switch → Fixed ATM Network. W-ATM switch handles handover, radio resource management.

• Major Research Challenges:

  1. Handover: Seamless cell transfer with QoS preservation.

  2. Error Resilience: Wireless BER high → need FEC, ARQ at sub-cell layer.

  3. Limited Bandwidth: Wireless link capacity << wired ATM.

  4. Power Constraints: For mobile terminals.

III. ADVANCED ANTENNA & MODULATION TECHNIQUES

A. SISO vs. MIMO Systems

[!TIP] MIMO Gain: Capacity scales linearly with min(N_t, N_r) under rich scattering. Diversity order increases with number of antennas.

B. OFDM & OFDM-MIMO

OFDM Principle:

• Parallel Transmission: High-rate data stream split into N low-rate subcarriers.

• Orthogonality: Subcarriers spaced by Δf = 1/T_sym (symbol duration) → no ICI (Inter-Carrier Interference).

• Implementation: IFFT at transmitter, FFT at receiver.

• Combat ISI: Add Cyclic Prefix (CP) longer than channel delay spread. CP is copy of end of symbol prepended.

  • CP Length T_cp: T_cp > τ_max (max multipath delay). Makes channel appear circular convolution, enabling simple frequency-domain equalization (one tap per subcarrier).

  • Overhead: CP reduces spectral efficiency by T_cp / (T_sym + T_cp).

OFDM-MIMO Synergy:

1. MIMO exploits frequency-selective fading of each OFDM subcarrier independently.

2. On each subcarrier, MIMO operates as flat-fading SISO/MIMO link.

3. Result: Combats both inter-symbol interference (via CP) and fading (via MIMO diversity/multiplexing) across frequency and space.

4. Channel Estimation: Pilots inserted in OFDM grid for per-subcarrier MIMO channel estimation.

\boxed{\text{OFDM converts frequency-selective fading channel into } N \text{ parallel flat-fading channels.}}

IV. WIRELESS SENSOR NETWORKS (WSN) & INTERNET OF THINGS (IoT)

A. WSN Fundamentals

Architecture:

[Sensor Nodes] → [Sink/Base Station] → [Gateway] → [Management Station/Internet]

       ↑

   (Ad-hoc Multi-hop)

• Sensor Node: Sensing, processing, communication, power. Constrained (energy, CPU, memory).

• Sink/Base Station: Collects data, may have more resources.

• Gateway: Connects WSN to external networks (Internet).

• Protocol Stack: Physical → MAC → Network (routing) → Transport → Application. Cross-layer design common.

Comparison with Wired Networks:

Key Applications: Environmental monitoring (temp, humidity), health care (patient monitoring), industrial automation, smart agriculture, military surveillance.

B. Underwater Wireless Sensor Networks (UWSN)

Unique Architecture:

• Node Types: Sensor nodes (bottom), gateway/surface buoy (relay to on-shore), autonomous underwater vehicles (AUVs).

• Communication Medium:

  • Acoustic: Primary (low bandwidth ~kbps, high delay ~1s/km, multipath, Doppler, limited range).

  • RF: Very short range (<1m), high attenuation.

  • Optical: Short range (<100m), requires line-of-sight, high data rate.

Main Challenges:

1. High Propagation Delay: ~1500 m/s vs. 3e8 m/s (RF) → large RTT.

2. Limited Bandwidth: Acoustic bandwidth narrow (10s of kHz).

3. Severe Multipath & Doppler: Long delay spread, moving nodes/water currents.

4. Node Mobility: Drift with currents → topology changes.

5. Energy Harvesting Difficult: Underwater environment harsh, battery replacement costly.

Applications: Oceanographic data collection, pipeline/cable monitoring, disaster prevention (tsunami detection), marine archaeology.

C. WSN Routing Protocols

Classification:

• Data-Centric (Query-based): e.g., SPIN (Sensor Protocols for Information via Negotiation - meta-data negotiation), Directed Diffusion (interest propagation, gradient setup).

• Hierarchical (Cluster-based): e.g., LEACH (Low-Energy Adaptive Clustering Hierarchy - randomized cluster-head rotation), TEEN (Threshold-sensitive Energy Efficient sensor Network - reactive to thresholds).

• Location-Based: Uses node location (GPS/coordinate) for geographic routing (e.g., GPSR).

• QoS-Aware: e.g., SPEED (ensure real-time delivery).

Proactive (Table-Driven) vs. Reactive (On-Demand):

D. WSN Topology Management

• Concept: Organizing nodes into clusters (hierarchical) or managing sleep/wake cycles to maintain connectivity with minimal energy.

• Essentiality:

  1. Energy Efficiency: Cluster-heads aggregate data, reduce long-hop transmissions. Sleep scheduling saves power.

  2. Robustness/Fault Tolerance: Redundant paths, cluster-head rotation prevents single point failure.

  3. Scalability: Hierarchical structure manages large networks.

  4. Coverage Optimization: Ensure sensing area is covered while some nodes sleep.

E. WSN Security

Challenges: Resource constraints (no heavy crypto), hostile deployment (physical capture), lack of infrastructure, insider attacks.

Techniques:

• Key Management: Symmetric key (pre-distribution, LEAP), asymmetric (ECC for low power).

• Secure Routing: Secure path establishment (authentication), avoiding compromised nodes.

• Data Confidentiality: Encryption (AES, lightweight ciphers).

• Data Integrity & Authenticity: MACs (Message Authentication Codes), hash chains.

• **Availability:**抵抗DoS (e.g., rate limiting, sleep discipline).

F. Internet of Things (IoT) Architecture

Layered Architecture:

1. Perception Layer: Sensors/actuators, data acquisition. (e.g., temperature sensor, RFID).

2. Network Layer: Connectivity. WSN, WPAN (Bluetooth, Zigbee), Cellular (NB-IoT, LTE-M), LPWAN (LoRaWAN). Gateways for protocol translation.

3. Middleware/Platform Layer: Data processing, storage, cloud platforms (AWS IoT, Azure IoT). Device management, data analytics.

4. Application Layer: User-facing apps (smart home, industrial IoT, smart city).

Key Components: Things (devices), Gateways, Cloud Platforms, Analytics Engines, User Interfaces.

Design Principles: Scalability (billions of devices), Interoperability (standard protocols), Security (end-to-end), Energy Efficiency (battery life years).

Emerging Standards:

• LoRaWAN: Long Range, low power WAN (chirp spread spectrum).

• NB-IoT: Narrowband IoT (LTE-based, licensed spectrum).

• MQTT: Lightweight publish/subscribe messaging protocol.

• CoAP: Constrained Application Protocol (RESTful for devices).

• 6LoWPAN: IPv6 over Low-Power WPAN (adapts IPv6 to 802.15.4).

V. MOBILITY MANAGEMENT & TRANSPORT LAYER PROTOCOLS

A. Mobility Management (General)

• Handover (Handoff): Transfer of ongoing session from one access point/base station to another.

  • Horizontal: Within same network (e.g., 4G cell to 4G cell).

  • Vertical: Between different networks (e.g., Wi-Fi to 4G).

• Location Management:

  • Registration (Location Update): Mobile reports its current location to network.

  • Paging: Network broadcasts to find mobile for incoming call.

• Roaming: Ability to use services outside home network (via agreements).

B. Mobile IP

Core Concepts:

• Home Agent (HA): Router in home network. Stores Care-of Address (CoA). Tunnels packets to CoA.

• Foreign Agent (FA): Router in visited network. Provides CoA (often its own address), forwards packets.

• Mobile Node (MN): Has permanent Home Address (HoA). When away, gets CoA (via FA or DHCP).

• Tunneling: Encapsulation of IP packet with destination=CoA. HA tunnels to FA/CoA, which decapsulates and delivers to MN.

Data Forwarding Process:

1. Triangular Routing: CN sends to MN's HoA → HA intercepts → tunnels to CoA → MN receives. Inefficient (packets go via HA even if CN and MN are in same foreign network).

2. Route Optimization:

   • MN sends Binding Update (BU) to CN with its current CoA.

   • CN caches binding (HoA ↔ CoA).

   • CN tunnels directly to CoA (bypassing HA). Reduces latency, avoids HA bottleneck.

   • HA still intercepts packets to update its cache.

[!TIP] Key Problem: Triangle routing causes extra hop, latency. Route Optimization solves this but requires CN support and security (BU authentication).

C. TCP for Wireless/Mobile Networks

Problems with Standard TCP:

• Packet Loss → Congestion: TCP interprets all loss as congestion → reduces window unnecessarily (wireless loss due to errors).

• High RTT: Large congestion window needed, slow to recover.

• Frequent Handovers: Connection breaks, timeouts, slow start.

TCP Variants & Enhancements:

D. IPv4 vs. IPv6 Addressing (Comparative)

VI. SPECIAL TOPICS & EMERGING SYSTEMS

A. GPS & Aided Navigation (GAGAN)

• GPS-Aided GEO Augmented Navigation (GAGAN): Indian SBAS (Satellite-Based Augmentation System). Uses geostationary satellites to broadcast correction signals.

• Purpose: Improve accuracy (from ~10m to <3m), integrity (warning of faulty signals), and availability (especially for aviation) over Indian region.

• Applications: CAT-I precision approach for aircraft, maritime, land-based.

B. Sensor Body Area Networks (BAN)

Case Study: Health Monitoring

• Architecture:

  
  [Wearable Sensors (ECG, SpO2, Temp)] → [Personal Server/Gateway (Smartphone)] → [Medical Server/Cloud] → [Doctor/Hospital]
  
  

• Sensors: On-body or implanted. Low-power, short-range (Bluetooth/BLE, Zigbee).

• Gateway: Aggregates data, local processing, connects to internet via Wi-Fi/3G/4G.

• Applications: Remote patient monitoring, elderly care, sports fitness, military (soldier monitoring).

C. Wireless ATM (Recap)

• Historical Research Topic: Aimed to bring ATM's QoS guarantees to wireless.

• Why Failed: Complexity, overhead of ATM cells (53 bytes) over error-prone wireless, rise of IP-based solutions (DiffServ, MPLS) and more efficient link-layer protocols (Wi-Fi, cellular).

• Legacy: Concepts influenced later QoS frameworks.

PRIORITY MAPPING & EXAM TIPS

[!CAUTION] Common Pitfalls:

1. Confusing UMTS & LTE components: UMTS has RNC, LTE has eNB (RNC eliminated). UMTS has separate CS/PS cores, LTE is all-IP.

2. MIMO Gain: Don't just say "more antennas." Specify multiplexing gain (rate), diversity gain (reliability), beamforming (coverage).

3. OFDM CP: Must be longer than channel delay spread (T_cp > τ_max) to eliminate ISI.

4. WSN Routing: Proactive = high overhead, low latency; Reactive = low overhead, high latency. Choose based on traffic pattern.

5. Mobile IP: Triangular routing is default; route optimization requires CN binding updates.

6. TCP Variants: Know the specific improvement of each (Tahoe=fast retransmit, Reno=fast recovery, Vegas=RTT-based, M-TCP=local retransmit + window freeze).

Final Formula Box:

• OFDM Subcarrier Spacing: Δf = 1 / T_sym (ensures orthogonality).

• MIMO Capacity (ideal): C = min(N_t, N_r) * B * log2(1 + SNR) (bps).

• CP Overhead: η = T_sym / (T_sym + T_cp) (spectral efficiency factor).